Small lump on my arm for several years

Patient

Q: I have had a very small lump on my right forearm for as long as I can remember (my memory goes back until I was about 3 years old). It doesn't hurt no matter what I do to it. I actually seem to feel less pain on that specific spot in my arm than the rest of my arm. It feels hard when I poke it, and I can sort of move it up and down my arm about half an inch in each direction. It goes back to it's original position when I let it go. It's not visible on my arm, but I can feel with the lightest touch.
I don't have any health related issues aside from seasonal allergies which I just acquired this Spring. When I was small I suffered from high levels of lead poisoning in my blood for about 7 years, but I haven't had any issues for the past 15 years. I haven't worried about it since its never been in the way and it doesn't feel dangerous, but I've been thinking that someday it could develop into something. What could it be?
Thank you so much in advance for your help!

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